This invention relates to a process for retorting synthetic fuels, and more particularly, to a fluid bed process for retorting oil shale, tar sands and other solid hydrocarbon-containing material.
Researchers have now renewed their efforts to find alternate sources of energy and hydrocarbons in view of recent rapid increases in the price of crude oil and natural gas. Much research has been focused on recovering hydrocarbons from solid hydrocarbon-containing material such as oil shale, coal and tar sands by pyrolysis or upon gasification to convert the solid hydrocarbon containing material into more readily usable gaseous and liquid hydrocarbons.
Vast natural deposits of oil shale found in the United States and elsewhere contain appreciable quantities of organic matter known as "kerogen" which decomposes upon pyrolysis or distillation to yield oil, gases and residual carbon. It has been estimated that an equivalent of 7 trillion barrels of oil are contained in oil shale deposits in the United States with almost sixty percent located in the rich Green River oil shale deposits of Colorado, Utah and Wyoming. The remainder is contained in the leaner Devonian-Mississippian black shale deposits which underlie most of the eastern part of the United States.
As a result of dwindling supplies of petroleum and natural gas, extensive efforts have been directed to develop retorting processes which will economically produce shale oil on a commercial basis from these vast resources.
Generally, oil shale is a fine-grained sedimentary rock stratified in horizontal layers with a variable richness of kerogen content. Kerogen has limited solubility in ordinary solvents and therefore cannot be recovered by extraction. Upon heating oil shale to a sufficient temperature, the kerogen is thermally decomposed to liberate vapors, mist, and liquid droplets of shale oil and light hydrocarbon gases such as methane, ethane, ethene, propane and propene, as well as other products such as hydrogen, nitrogen, carbon dioxide, carbon monoxide, ammonia, steam and hydrogen sulfide. A carbon residue typically remains on the retorted shale.
Shale oil is not a naturally occurring product, but is formed by the pyrolysis of kerogen in the oil shale. Crude shale oil, sometimes referred to as "retort oil," is the liquid oil product recovered from the liberated effluent of an oil shale retort. Synthetic crude oil (syncrude) is the upgraded oil product resulting from the hydrogenation of crude shale oil.
The process of pyrolyzing the kerogen in oil shale, known as retorting, to form liberated hydrocarbons, can be done in surface retorts in aboveground vessels or in in situ retorts under ground. In principle, the retorting of shale and other hydrocarbon-containing materials, such as coal and tar sands, comprises heating the solid hydrocarbon-containing material to an elevated temperature and recovering the vapors and liberated effluent. However, as medium grade oil shale yields approximately 20 to 25 gallons of oil per ton of shale, the expense of materials handling is critical to the economic feasibility of a commercial operation.
In order to obtain high thermal efficiency in retorting, carbonate decomposition should be minimized. Colorado Mahogany zone oil shale contains several carbonate minerals which decompose at or near the usual temperature attained when retorting oil shale. Typically, a 28 gallon per ton oil shale will contain about 23% dolomite (a calcium/magnesium carbonate) and about 16% calcite (calcium carbonate), or about 780 pounds of mixed carbonate minerals per ton. Dolomite requires about 500 BTU per pound and calcite about 700 BTU per pound for decomposition, a requirement that would consume about 8% of the combustible matter of the shale if these minerals were allowed to decompose during retorting. Saline sodium carbonate minerals also occur in the Green River formation in certain areas and at certain stratigraphic zones. The choice of a particular retorting method must therefore take into consideration carbonate decomposition as well as raw and spent materials handling expense, product yield and process requirements.
In surface retorting, oil shale is mined from the ground, brought to the surface, crushed and placed in vessels where it is contacted with a hot solid heat carrier material, such as hot spent shale, ceramic balls, metal balls, or sand or a gaseous heat carrier material, such as light hydrocarbon gases, for heat transfer. The resulting high temperatures cause shale oil to be liberated from the oil shale leaving a retorted, inorganic material and carbonaceous material such as coke. The carbonaceous material can be burned by contact with oxygen at oxidation temperatures to recover heat and to form a spent oil shale relatively free of carbon. Spent oil shale which has been depleted in carbonaceous material is removed from the retort and recycled as heat carrier material or discarded. The combustion gases are dedusted in cyclones or electrostatic precipitators.
Some well-known processes of surface retorting are: N-T-U (Dundas Howes retort), Kiviter (Russian), Petrosix (Brazilian), Lurgi-Ruhrgas (German), Tosco II, Galoter (Russian), Paraho, Koppers-Totzek, Fushan (Manchuria), Union, gas combustion and fluid bed. Process heat requirements for surface retorting processes may be supplied either directly or indirectly.
Directly heated surface retorting processes, such as the N-T-U, Kiviter, Fushan and gas combustion processes, rely upon the combustion of fuel, such as recycled gas or residual carbon in the spent shale, with air or oxygen within the bed of shale in the retort to provide sufficient heat for retorting. Directly heated surface retorting processes usually result in lower product yields due to unavoidable combustion of some of the products and dilution of the product stream with the products of combustion. The Fushan process is shown and described at pages 101-102, in the book Oil Shales and Shale Oils, by H. S. Bell, published by D. Van Norstrand Company (1948). The other processes are shown and described in the Synthetic Fuels Data Handbook, by Cameron Engineers, Inc. (Second Edition, 1978).
Indirectly heated surface retorting processes, such as the Petrosix, Lurgi-Ruhrgas, Tosco II and Galoter processes, utilize a separate combustor or heater for heating solid or gaseous heat-carrying material which is injected, while hot, into the shale in the retort to provide sufficient heat for retorting. Generally, indirect heating surface retorting processes result in higher yields and less dilution of the retorting product than directly heated surface retorting processes, but at the expense of additional solids handling.
Fluid bed surface retorting processes are particularly advantageous. The use of fluidized bed contacting zones has long been known in the art and has been widely used in fluid catalytic cracking of hydrocarbons. When a fluid is passed at a sufficient velocity upwardly through a contacting zone containing a bed of subdivided solids, the bed expands and the particles are buoyed and supported by the drag forces caused by the fluid passing through the interstices among the particles. The superficial vertical velocity of the fluid in the contacting zone at which the fluid begins to support the solids is known as the "minimum fluidization velocity." The velocity of the fluid at which the solid becomes entrained in the fluid is known as the "terminal velocity" or "entrainment velocity." Between the minimum fluidization velocity and the terminal velocity, the bed of solids is in a fluidized state and it exhibits the appearance and some of the characteristics of a boiling liquid. Because of the quasi-fluid or liquid-like state of the solids, there is typically a rapid overall circulation of all the solids throughout the entire bed with substantially complete mixing, as in a stirred-tank reaction system.
Typifying those prior art fluidized bed retorting processes, fluid catalytic cracking processes, transfer line processes and similar processes are the Union Carbide/Battelle coal gasification process, the fluid coker and flexicoking processes described at page 300 of the Synthetic Fuels Data Handbook, by Cameron Engineers, Inc. (Second Edition, 1978) and those found in U.S. Pat. Nos. 2,471,119; 2,506,307; 2,518,693; 2,608,526; 2,657,124; 2,626,234; 2,634,233; 2,643,218; 2,643,219; 2,684,931; 2,793,104; 2,799,359; 2,807,571; 2,844,525; 3,039,955; 3,152,245; 3,281,349; 3,297,562; 3,499,834; 3,501,394; 3,617,468; 3,663,421; 3,703,052; 3,803,021; 3,803,022; 3,855,070; 3,867,110; 3,890,111; 3,891,402; 3,929,585; 3,976,558; 3,980,439; 4,052,172; 4,064,018; 4,087,347; 4,110,193; 4,125,453; 4,133,739; 4,137,053; 4,141,794; 4,148,710; 4,152,245; 4,157,245; 4,183,800; 4,199,432; 4,210,491; and 4,247,987, as well as in U.S. patent applications, Ser. Nos. 293,694 filed Aug. 17, 1981, 333,039 filed Dec. 21, 1981, and 333,040 filed Dec. 21, 1981. Cooling of spent shale is described in U.S. patent application, Ser. No. 208,163 filed Nov. 19, 1980. These prior art processes have met with varying degrees of success.
A problem with some prior art fluidized bed processes is the long residence time at high temperatures which results in many secondary and undesirable side reactions such as thermal cracking, which usually increases the production of less desirable gaseous products and decreases the yield and quality of desirable condensable products. Therefore, in any process designed to produce the maximum yield of high quality condensable hydrocarbons, it is preferred that the volatilized hydrocarbons are quickly removed from the retorting vessel in order to minimize deleterious side reactions such as thermal cracking.
Another problem with many prior art processes, particularly with countercurrent fluidized bed flow processes, using solid heat carrier material as the heat source, is that after the shale oil has been vaporized, it then comes in contact with countercurrent flowing solids which are at a much cooler temperature, which leads to condensation of a portion of the shale oil and reabsorption of a portion of the vaporized shale oil into the downward flowing shale. This condensation and reabsorption leads to coking, cracking and polymerization reactions, all of which are detrimental to producing the maximum yield of condensable hydrocarbons.
A further problem with many prior art fluidized bed processes is that they often have low lateral mixing and high backmixing resulting in poor plug flow, and excessive bed volumes. Moreover, many prior art fluidized bed processes require high fluidizing velocities and pressures.
It is therefore desirable to provide an improved fluid bed retorting process whichever comes most, if not all, of the preceding problems.